(1) Field of the Invention
The present invention relates to an image forming apparatus with a charging member which charges an image bearer with a photoconductive surface, and preferably relates to an image forming apparatus having a charging member in which a ferroelectric layer subjected to a dipole orienting treatment (poling treatment) is formed on a surface opposing to the image bearer, such as a copier, laser beam printer and other image recording apparatus including liquid development process.
(2) Description of the Prior Art
Generally, an electrophotographic image forming apparatus such as a copier, laser printer or the like comprises seven processing units as shown in FIG. 6: a photosensitive member 101 as an image bearer; a charger 102 for charging the photosensitive member 101; an exposure unit 103 for forming a latent image by light exposure; a developing unit 104 for performing development with toner; a transfer device 105 for transferring the toner image to a transfer medium; a fixing device (not shown) for fixing the toner image on transfer medium; an erasing device 106 for erasing charge on the photosensitive member; and a cleaning unit 107 for removing the leftover toner from the photosensitive member.
In recent years, various contact type charging devices have been developed in place of corona chargers in order to provide an ozoneless, low-cost, compact, and energy saving configuration for the charging member 102. In this contact type charging device, the charging member applied with a voltage is set in abutment with the photosensitive member so that the photosensitive member surface is charged by a discharge phenomenon or the like, and a charging roller type in which a conductive roller is used as a charging member is preferable in terms of the stability of electrification.
Since a charging phenomenon is conducted by discharging from the charging member to the photosensitive member, electrification is started by applying a voltage equal to or higher than the threshold voltage by a voltage power supply 108. For example, when a charging roller is pressurized to contact with an OPC photosensitive member with the thickness of 25 xcexcm and applied with a voltage of about 700V or higher, the surface potential of the photosensitive member starts to increase, thereafter the surface potential of the photosensitive member linearly increases with the applied voltage with a gradient of 1.
Hereinbelow, the threshold voltage is defined as the electrification start voltage Vth. That is, in order to obtain the surface potential of the photosensitive member VoPc necessary for electrophotography, the charging roller needs a DC voltage equal to (VoPc+Vth) or higher. The charging roller has a roller configuration made of a metal core of aluminum, iron or the like, which is covered with an electrically conductive tubular elastomeric element or an electrically insulative tubular elastomeric element (polyurethane, EPDM, silicone rubber, NBR, etc. ) in which a conductor (ionic conductors, carbon black, metal oxides, metal powders, graphite, etc.) is dispersed. This roller (to be referred to hereinbelow as xe2x80x9ccharging rollerxe2x80x9d) is set in abutment with the photosensitive member surface and a bias voltage of +(xe2x88x92)500 V or higher is applied to the metal core, or the DC bias component superimposed with an AC bias component, for example 1.6 kVpp, is applied if necessary, so that the surface of the photosensitive member is uniformly charged at about +(xe2x88x92) 600V.
However, the conventional charging means, wherein a bias voltage is applied to the metal core of the charging roller, requires a bias application means, therefore a high-voltage power supply is needed, which leads to increase in the cost of the apparatus, increase in apparatus size for installing the power source, increase in consumption of power and increase in the number of consumable parts, results in inconsistency with regard to energy saving and ecologically-oriented development, which have become increasingly important for manufactures.
Therefore, in order to obtain a charging device having no need for a high-voltage power supply, as disclosed in Japanese Patent Application 48923/1999, a conductive roll support structure with a pyroelectric film layer is set into contact with a photoconductive surface member, and the pyroelectric film is provided with a heater which contacts with the pyroelectric film to heat it. Thereby, the pyroelectric film is heated, and by heating and cooling it, thermal expansion or thermal contraction occurs, thereby, the surface charge density is changed. Using this change, the pyroelectric potential is generated on the pyroelectric film for charging the photoconductive member as needed before exposing the photoconductive member. As above, a method was proposed, wherein using the piezoelectric effect of the pyroelectric film the pyroelectric potential is generated in the pyroelectric film, thereby, the photoconductive surface is charged.
However, even if the method in Japanese Patent Application 48923/1999 is used, in order to generate the pyroelectric potential by the pyroelectric film, it is necessary to set a heating mechanism for heating the pyroelectric film, furthermore, heating needs to be carried out by the heating mechanism, which thereby leads to an increase in consumption of power and it does not successfully achieve the essential improvement in regards to energy-saving, low-cost, downsizing, etc.
The present invention has been achieved in order to solve the above problems with the conventional technology, it is therefore an object of the present invention to provide an image forming apparatus capable of realizing low-cost and reduction of the number of consumable parts by downsizing a charger and reducing power consumption thereof.
The inventors hereof have earnestly studied and as a result, have successfully completed the invention by developing a process for uniformly charging the photosensitive member surface, by using a ferroelectric for the charging device in an electrophotographic process for applying the electric field formed by permanent dipoles in a ferroelectric. That is, the photosensitive member surface is uniformly electrified by the electric field formed by permanent dipoles of the ferroelectric, which conducts a different process from the conventional one, and the charging device does not need to be provided with a high-voltage power supply for applying a bias potential for electrification. Thus, energy-saving, low-cost and downsizing in the charging device was successfully realized.
Therefore, in a charging member for electrifying the surface of a photosensitive member which is an image bearer, by forming a layer, for example a ferroelectric layer, subjected to a dipole orienting treatment (poling treatment) on the surface layer of the charging member which is contacted with the photosensitive member, and developing a new electrifying process for electrifying uniformly the photosensitive member surface by the function of the electric field formed by dipoles in the charging member, an image forming apparatus is provided to achieve the above object.
The present invention for attaining the above object is configured as the following aspects from 1 to 19:
In accordance with the first aspect of the present invention, an image forming apparatus with a charging member for electrifying an image bearer which has a photoconductive surface is characterized in that the charging member is arranged opposing to the image bearer and has a layer containing the ferroelectric at least as part, the ferroelectric is subjected to a dipole orienting treatment in advance, wherein the photoconductive surface of the image bearer is electrified by electric field formed by the dipoles of the ferroelectric.
In accordance with the second aspect of the present invention, the image forming apparatus having the above first aspect is characterized in that the bias voltage applying means for electrifying is not provided to the charging member.
In accordance with the third aspect of the present. invention, the image forming apparatus having the above first or second aspect is characterized in that the charging member is constructed such that the ferroelectric layer is formed on an electrically conductive support.
In accordance with the fourth aspect of the present invention, the image forming apparatus having the above third aspect is characterized in that the electrically conductive support is grounded.
In accordance with the fifth aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that the polarity of the ferroelectric layer is set positive when the toner on the image bearer is charged negative and the polarity of the ferroelectric layer is set negative when the toner on the image bearer is charged positive.
In accordance with the sixth aspect of the present invention, the image forming apparatus having any one of the above first through fourth aspects is characterized in that the thickness of the ferroelectric layer is 24 xcexcm or greater.
In accordance with the seventh aspect of the present invention, the image forming apparatus having any one of the above first through sixth aspects is characterized in that the ferroelectric layer includes at least an organic material as part thereof.
In accordance with the eighth aspect of the present invention, the image forming apparatus having the above seventh aspect is characterized in that the organic material is poly vinylidene fluoride-tetrafluoroethylene copolymer [P(VDF-TeFE)].
In accordance with the ninth aspect of the present invention, the image forming apparatus having the above seventh aspect is characterized in that the organic material is poly vinylidene fluoride-trifluoroethylene copolymers [P(VDF-TrFE)].
In accordance with the tenth aspect of the present invention, the image forming apparatus having any one of the above first through sixth aspects is characterized in that the ferroelectric at least includes an inorganic material as part thereof.
In accordance with the eleventh aspect of the present invention, the image forming apparatus having the above tenth aspect is characterized in that the inorganic material is a ceramics sintered compact composed of at least three components which are given as a general form of [(Bi2O2)2+(XY2O7)2xe2x88x92] or given in a general form of [XnBi4Tin+3O3n+12] where X represents Sr, Pb, Ba or Na0.5 Bi0.5, Y represents Ta or Nb, and n represents 1 or 2.
In accordance with the twelfth aspect of the present invention, the image forming apparatus having the above eleventh aspect is characterized in that the ceramics sintered compact are composed of bismuth-strontium titanate.
In accordance with the thirteenth aspect of the present invention, the image forming apparatus having any one of the above first through sixth aspects is characterized in that an abrasive-resistant material covers or coats the surface layer of the ferroelectric.
In accordance with the fourteenth aspect of the present invention, the image forming apparatus having any one of the above first through sixth aspects is characterized in that the relative permittivitty es of the ferroelectric is set equal to or greater than 10.
In accordance with the fifteenth aspect of the present invention, the image forming apparatus having any one of the above first through sixth aspects is characterized in that the volume resistivity of the ferroelectric falls within the range from 1014xcexa9xc2x7cm to 1015xcexa9xc2x7cm.
In accordance with the sixteenth aspect of the present invention, the image forming apparatus having any one of the above first through fifteenth aspects characterized in that the volume resistivity of the conductive support substrate is set to be equal to or lower than 106xcexa9xc2x7cm.
In accordance with the seventeenth aspect of the present invention, the image forming apparatus having any one of the above first through sixteenth aspect is characterized in that the volume resistivity of the ferroelectric is set to be equal to or lower than 1012xcexa9xc2x7cm when it is heated within the range below the Curie temperature.
In accordance with the eighteenth aspect of the present invention, the image forming apparatus having any one of the above first through seventeenth aspects is characterized in that the following relationship holds:
Lxe2x89xa7Vp/Vopc
where Vp(V/xcexcm) represents the pyroelectric potential L(xcexcm) represents the thickness of the ferroelectric layer, and Vopc (V) represents the charged potential of the image bearer.
In accordance with the nineteenth aspect of the present invention, the image forming apparatus having any one of the above first through eighteenth aspects is characterized in that the following relationship holds:
L greater than {Vopc+312+6.2(Lp/xcex5sP)}/{Vpxe2x88x92(6.2/xcex5s)}
where Lp(xcexcm) represents the thickness of the image bearer, xcex5sP represents the relative permittivity of the image bearer, Vopc(V) represents charged potential of the image bearer, Vp(V/xcexcm) represents the pyroelectric potential appearing per unit thickness of the ferroelectric layer, L(xcexcm) represents the thickness of the ferroelectric layer, and xcex5s represents the relative permittivity of the ferroelectric.